1. Field of the Invention
The present invention relates to a system and method for reducing emissions of nitrogen oxides (NOx) and particulate matter (PM) in a compression ignition engine.
2. Background Art
Internal combustion engines, and in particular, compression ignition (or diesel) engines have a wide variety of applications including passenger vehicles, marine vessels, earth-moving and construction equipment, stationary generators, and on-highway trucks, among others. Such applications typically have a variety (e.g., light, medium, and heavy) of operating conditions (i.e., engine modes).
When the temperature of the engine combustion chamber exceeds a particular temperature in relation to the air/fuel equivalence ratio, oxides of nitrogen (i.e., nitrogen oxides, NOx) and particulate matter (PM) may be generated as byproducts of the combustion process. NOx and PM are generally considered undesirable pollutants and, in particular, are typically believed to be factors in the formation of smog.
Referring to FIG. 1, a diagram 10 illustrating published data indicating local temperature and local equivalence ratios that are known to promote formation of NOx (i.e., region 106) and PM (i.e., region 104) in diesel engines is shown. The diagram 10 further shows a Low Temperature High Equivalence Ratio Combustion region (LTHE) 12, and a Low Temperature Low Equivalence Ratio region (LTLE) 14. The LTHE region 12 is characterized by in-cylinder temperatures in the range of 1000 K to 1650 K, and combustion equivalence ratios in the range of 0 to 8. The LTLE region 14 is characterized by in-cylinder temperatures in the range of 1000 K to 2000 K, and combustion equivalence ratios in the range of 0 to 0.5. Conventional approaches to limiting the formation of NOx and PM have focused on operating the engine within regions 12 or 14, or a combination of regions 12 and 14. While an engine operating within either the LTHE 12 or LTLE 14 regions may produce low NOx and PM emissions, such operation has the deficiency of limiting the engine to light load applications. Furthermore, operating within regions 12 or 14 may also produce excessive penalties in fuel consumption and in hydrocarbon and carbon monoxide emissions.
Therefore, it would be desirable to have a system and method for reducing the formation of NOx and PM for compression ignition combustion engines that reduces or overcomes the deficiencies of conventional approaches.